The term "groundwater" applies to water that occurs below the surface of the earth, where it occupies all or part of the void spaces in soils or geologic strata. It is also called subsurface water to distinguish it from surface water which is found in large bodies like the oceans or lakes, or which flow overland in streams.
Most groundwater comes from infiltration of precipitation. Precipitation infiltrates below the ground surface into the soil zone. There is a zone of aeration, or "vadose zone," where the interstices are occupied partially by water and partially by air. When the soil zone becomes partially saturated, water can percolate downward. A zone of saturation occurs where all the interstices are filled with water. Groundwater continues to descend until, at some depth, it merges into a zone of dense rock. Water is contained in the pores of such rocks, but the pores are not connected and water will not migrate. The upper limit of the portion of the ground wholly saturated with water is known as the water table.
Groundwater is constantly in motion. Compared to surface water, it moves very slowly, the actual rate dependent on the transmissivity and storage capacity of the aquifer. Natural outflows of groundwater take place through springs and riverbeds when the groundwater pressure is higher than atmospheric pressure in the vicinity of the ground surface. Internal circulation is not easily determined, but near the water table the average cycling time of water may be a year or less, while in deep aquifers it may be as long as thousands of years.
Groundwater plays a vital role in the development of arid and semiarid zones, sometimes supporting vast agricultural and industrial enterprises that could not otherwise exist. A vast amount of groundwater is distributed throughout the world, and a large number of groundwater reservoirs are still undeveloped or uninvestigated.
The vadose zone is a region of aeration above the water table. This zone also includes the capillary fringe above the water table, the height of which will vary according to the grain size of the sediments. In coarse-grained mediums the fringe may be flat at the top and thin, whereas in finer grained material it will tend to be higher and may be very irregular along the upper surface. The vadose zone varies widely in thickness, from being absent to many hundreds of feet, depending upon several factors. These include the environment and the type of earth material present. Water within this interval, which is moving downward under the influence of gravity, is sometimes called vadose water, or gravitational water. In the following description, it shall be referred to as "soil water"
Soil water pressure data from monitoring wells are used for various purposes. The term "well" is intended to encompass boreholes used with tensiometers. Monitored soil water pressure data is used to determine the magnitude and direction of hydraulic gradient at underground storage tanks sites, remedial investigation sites, and other sites effected by local and federal environmental laws and regulations.
The soil provides a major reservoir for water within a catchment. Soil moisture increases when there is sufficient rainfall to exceed losses to evaporation, transpiration and drainage to groundwater and streams. It is depleted during the summer when evaporation and ranspiration rates are high. Levels of soil moisture are important for plant and crop growth, soil erosion, and slope stability. The moisture status of the soil is expressed in terms of a volumetric moisture content and the capillary potential of the water held in the soil pores. As the soil becomes wet, the water is held in larger pores, and the capillary potential increases.
Capillary potential may be measured by using a tensiometer consisting of a waterfilled porous cup connected to a manometer or pressure transducer. Soil moisture content is often measured gravimetrically by drying a soil sample under controlled conditions, though there are now available moisture meters based on the scattering of neutrons, dielectric properties of water or absorption of gamma rays from a radioactive source.
The rate at which water flows through soil is dependent on the gradient of hydraulic potential (primarily the sum of capillary potential and elevation) and the physical properties of the soil expressed in terms of a parameter called hydraulic conductivity, which varies with soil moisture in a nonlinear way. Measured sample values of hydraulic conductivity have been shown to vary rapidly in space, making the use of measured point values for predictive purposes at larger scales subject to some uncertainty.
Water also moves in soil because of differences in temperature and chemical concentrations of solutes in soil water. The latter, which can be expressed as an osmotic potential, is particularly important for the movement of water into plant roots due to high solute concentrations within the root water.
Changes in atmospheric pressure (barometric pressure) might cause soil water pressure (if unsaturated) or level (if saturated) to rise and fall within the wells. The barometric pressure changes enter into soil water potential measurements because the air pressure at the depth of a monitoring tensiometer sensor might be different than the air pressure at the related land surface elevation conventionally used for reference purposes. This phenomena has been explained by several authors as it relates to groundwater measurements and is noted as an error inherent to measuring soil water conditions.
Variations in soil water potential or pressure due to barometric pressure effects have the potential to give false readings. This can result in miscalculations of items such as hydraulic gradients and flow directions, points of exposure, aquifer properties, and time to exposure from contaminated sites.
The effects of barometric fluctuations on groundwater tables are well documented. Barometric pressure changes can cause changes of up to one foot in measured water level versus actual water level. Barometric pressure fluctuations in the atmosphere can significantly impact water table levels within wells.
Errors due to barometric pressure changes are also seen in advanced tensiometer data. Several numerical solutions to adjust for effects of barometric pressure changes when using a tensiometer in a vadose zone have been proposed. Most of these corrections are inadequate because of a time lag between a pressure change in the atmosphere and the pressure in the soil column immediately above the water table. This affects the determination of direction of water flow and the calculated rate of water travel. The failure to provide "real time" corrections when using a tensiometer can make it difficult to recognize trends in soil water potential measurements.
Existing numerical solutions further require knowledge of soil/air diffusivity between the land surface above the well and the water table. The diffusivity changes with water content which, in turn, changes over time.
All of the existing numerical solutions are inadequate because there is a time lag between the pressure change of the atmosphere and the pressure applied to the soil water within a soil column immediately above the water table. This invention provides an engineering solution to the problem by routing the soil gas pressure adjacent to the porous cup of a tensiometer to its reference port on the backside of the pressure transducer, thereby correcting the data to true soil water potential. The resulting corrections are applicable to standard, advanced, or deep tensiometers when used in a vadose zone. The vadose zone isobaric well design provides data which does not include the error introduced by transient changes in barometric pressure. Barometric pressure changes are compensated automatically and on a "real time" basis, thereby saving considerable time and money in analyzing data. Removal of the barometric pressure effects on the data also allows for detection of soil water movement that previously could not be detected at sites such as waste disposal facilities.